CDMA frequency allocation

ABSTRACT

A Code Division Multiple Access (CDMA) communication system which allocates different sets of frequencies to cells with different transmission power levels. Based upon the transmission power levels of a base station for each cell, each base station is assigned to one of at least two groups of base stations. Each group of base stations is assigned a set of frequencies for traffic communication. The set of frequencies assigned to one group of base stations does not overlap with the set of frequencies assigned to a different group of base stations.

FIELD OF THE INVENTION

The present invention relates to the use of Code Division MultipleAccess (CDMA) communications techniques used in cellular radio telephonecommunication systems, and more particularly, to the allocation offrequencies to cells with different transmission power levels in a CDMAcommunication system.

BACKGROUND OF THE INVENTION

The cellular telephone industry has made phenomenal strides incommercial operations in the United States as well as the rest of theworld. Growth in major metropolitan areas has far exceeded expectationsand is outstripping system capacity. If this trend continues, theeffects of rapid growth will soon reach even the smallest markets.Innovative solutions are required to meet these increasing capacityneeds as well as to maintain high quality service and avoid risingprices.

Throughout the world, one important step in cellular systems is tochange from analog to digital transmissions. Equally important is thechoice of an effective digital transmission scheme for implementing thenext generation of cellular technology. Furthermore, it is widelybelieved that the first generation of personal communication networks(PCN) (employing low cost, pocket-sized, cordless telephones that can becarried comfortably and used to make and receive calls in the home,office, street, car, etc.), would be provided by the cellular carriersusing the next generation digital cellular system infrastructure and thecellular frequencies. The key feature demanded in these new systems isincreased traffic capacity.

Currently, channel access is achieved using frequency division multipleaccess (FDMA) and time division multiple access (TDMA) methods. In FDMA,a communication channel is a single radio frequency band into which asignals transmission power is concentrated. Interference with adjacentchannels is limited by the use of bandpass filters which only passsignal energy within the specified frequency band. Thus, with eachchannel being assigned a different frequency, system capacity is limitedby the available frequencies as well as by limitations imposed bychannel radios.

In TDMA systems, a channel consists of a time slot in a period train oftime intervals over the same frequency. Each period of time slots iscalled a frame. A given signal's energy is confined to one of these timeslots. Adjacent channel interference is limited by the use of a timegate or other synchronization element that only passes signal energyreceived at the proper time. Thus, the portion of the interference fromdifferent relative signal strength levels is reduced.

Capacity in a TDMA system is increased by compressing the transmissionsignal into a shorter time slot. As a result, the information must betransmitted at a correspondingly faster bit rate which increases theamount of occupied spectrum proportionally.

With FDMA or TDMA systems, or a hybrid FDMA/TDMA system, the goal is toensure that two potentially interfering signals do not occupy the samefrequency at the same time. In contrast, CDMA allows signals to overlapin both time and frequency. Thus, all CDMA signals share the samefrequency spectrum. In either the frequency or the time domain, themultiple access signals appear to be on top of each other.

In principle, the information data stream to be transmitted is firstcoded or spread and then combined with a long PN-sequence or a shorterscrambling-sequence. In the latter case, the scrambling-sequences areplanned from cell to cell so that neighboring cells use differentscrambling-sequences or scrambling-masks. The information data streamand the PN-sequence or the scrambling sequence can have the same ordifferent bit rates. The informational data stream and the PN-sequenceor the scrambling-sequence are combined by multiplying the two bitstreams together. Each information data stream or channel is allocated aunique spreading code. A plurality of coded information signals aretransmitted on radio frequency carrier waves and jointly received as acomposite signal at a receiver. Each of the coded signals overlaps allof the other coded signals, as well as noise related signals, in bothfrequency and time. By correlating the composite signal with one of theunique codes, a corresponding information signal is isolated anddecoded.

There are a number of advantages associated with CDMA communicationtechniques. The capacity limits of CDMA based cellular systems areprojected to be up to 20 times that of existing analog technology as aresult of the properties of a wideband CDMA system, such as improvedcoding gain/modulation density, voice activating gating, sectorizationand reuse of the same spectrum in every cell. CDMA transmission of voiceby a high bit rate decoder ensures superior, realistic voice quality.CDMA also provides for variable data rates allowing many differentgrades of voice quality to be offered. The scrambled signal format ofCDMA completely eliminates cross-talk and makes it very difficult andcostly to eavesdrop or track calls, ensuring greater privacy to callersand greater immunity from air-time fraud.

Despite the numerous advantages offered by CDMA, problems can occur whenthe CDMA system contains different size cells which have different powerlevels. One problem is how to allocate frequencies between the differenttypes of cells. While this problem can be easily handed in traditionalFDMA or TDMA systems, the problem is quite serious in CDMA systemsbecause all of the frequencies are used throughout the system. Thisproblem occurs, for example, when microcells are used within umbrellacells and at the border between urban and rural areas where differentsize cells are used. The general problem is that the uplink (from mobilestation to base station) and downlink (from base station to mobilestation) handoff points are not located at the same place. The downlinkhandoff point is located closer to the microcell than the uplink handoffpoint.

FIG. 1 illustrates a typical scenario of the use of microcells withinumbrella cells. An umbrella cell 10 contains a base station 12 and aplurality of microcells 14. Each microcell 14 contains a base station16. In this example, a mobile station 18 is located near the umbrellabase station 12 but is located in a microcell 14. The base station foran umbrella cell 10 generally operates at a power level which is muchhigher than the power level used for base stations of a microcell. Sincethe mobile station is located in the microcell 14 and is incommunication with the base station 16, the high powered signals fromthe umbrella base station 12 may interfere with the communicationsbetween the mobile station 18 and the microcell base station 16. Sincethe umbrella base station is operating at a high power level, theinterfering signals can easily be 10-20 decibels above the communicationsignal between the mobile station and the microcell base station. Evenif the processing gain of the CDMA system is large enough to handle suchinterfering signals, the capacity of the system will be decreased.Furthermore, if the mobile station 18 were to connect to the umbrellacell base station 12, the mobile station 18 would have to increase itspower which would interfere with the microcell base station 16 in theuplink direction.

FIG. 2 illustrates the problems that can occur around the border betweensmall (urban) and large (rural) cells. A rural cell 20 contains a basestation 22 and an urban cell 24 contains a base station 26. In thisexample, a mobile station is located near the border between the urbancell 24 and the rural cell 20. When the mobile station is moving in thedirection of the arrow A, the question becomes to which cell does themobile station belong. If the mobile station is connected to the basestation 26 in the urban cell 24, the mobile station may encounter aninterference from the signals from the rural base station 22 due to thedifferent power levels between the rural and urban base stations. If themobile station is connected to the base station 22 in the rural cell 20,the mobile station will have to increase the power of its owntransmission in order to adequately communicate with the base station22. As a result, the mobile station's transmission will interfere withthe reception of the base station 26 since the mobile station is closerto that base station.

SUMMARY OF THE INVENTION

The present invention overcomes the above-mentioned problems byallocating different frequencies or sets of frequencies to cells withdifferent transmission power levels. For example, umbrella cells areassigned one set of frequencies to operate on, while microcells areassigned a different set of frequencies to operate on. The sets offrequencies should be sufficiently separated in order to decreaseinterference between different cells.

In order to allow the communication system to use Mobile AssistedHandoff (MAHO), the present invention includes several other features.In one embodiment of the present invention, all base stations in thecommunication system transmit a known pilot-sequence. Each base stationtransmits a pilot-sequence on each of the frequencies assigned to theparticular base station as well as some or all of the other frequenciesused in the communication system which are not assigned to thatparticular base station.

Since the pilot-sequence signals will be received at each mobile stationalong with other signals containing speech information and noise, eachmobile station will have to use a subtractive demodulation process tosort through all of the received signals to detect and decode thedesired signals. Using the subtractive demodulation process, a mobilestation can detect and decode the information signals being sent to thatparticular mobile station as well as detect and decode thepilot-sequence signals broadcast from various base stations in thecommunication system. As a result, each mobile station can receive itsown signal on its own frequency and simultaneously measure the signalstrength of the neighboring base stations by measuring the signalstrength of their pilot-sequences which are being broadcasted on thesame frequency.

In another embodiment of the present invention, the present inventiontakes advantage of discontinuous transmission (DTX) and discontinuousreception (DRX) to measure the signal strength of neighboring basestations. It is well known that the capacity of a DS-CDMA system can beapproximately doubled if a transmission occurs only when there isinformation to be transmitted. In other words the transmitter is turnedoff during a pause in speech. If the transmitter before being turned offtransmits a message which tells the receiver that the transmission willbe discontinued for a certain period of time, the receiver can use therest of the time period to measure signal strengths of other signals onother frequencies. As a result, a mobile station can measure the signalstrength of other base stations.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail withreference to the preferred embodiments of the invention, giving only byway of example and illustrated in the accompanying drawings in which:

FIG. 1 illustrates a typical arrangement of umbrella cells inmicrocells;

FIG. 2 illustrates a communication system with rural and urban cells;

FIG. 3 illustrates a base station with a pilot-sequence signal in oneembodiment of the present invention;

FIG. 4 shows a series of graphs illustrating how CDMA signals aregenerated;

FIGS. 5 and 6 show a series of graphs for illustrating how CDMA signalsare decoded;

FIG. 7 shows a series of graphs illustrating CDMA subtractivedemodulation according to one embodiment of the present invention;

FIG. 8 illustrates a block diagram of a CDMA communications system of atype in which the present invention can be advantageously employed;

FIG. 9 illustrates typical speech activity in the present invention; and

FIG. 10 illustrates a speech frame for one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the following description is in the context of cellularcommunication systems involving portable or mobile radio telephonesand/or personal communication networks, it will be understood by thoseof ordinary skill in the art that the present invention may be appliedto other communication applications.

In the present invention, different sets of frequencies are assigned tocells with different transmission power levels. For example, microcellscan be assigned a set of frequencies which is different from the set offrequencies assigned to an umbrella cell. Furthermore, the differentsets of frequencies should be separated by a frequency band which islarge enough to decrease interference in the system. As a result, theinterference caused by the powerful signals generated in the umbrellacell will be decreased since the mobiles operating in the microcellswithin the umbrella cell will be using frequencies that are differentfrom the frequencies used by the umbrella cell.

However, assigning different sets of frequencies to different types ofcells has a drawback in that Mobile Assisted Handoff (MAHO) is notpossible. Mobile Assisted Handoff is not possible since mobiles arecontinuously receiving signals on their own frequency so they are unableto simultaneously measure the signal strength of other signals on otherfrequencies.

In one embodiment of the present invention, all of the base stations inthe communication system transmit a known pilot-sequence. Thepilot-sequence is transmitted just like any other type of channel, i.e.,a traffic channel. However, the pilot sequence may contain noinformation or a limited amount of information. The pilot-sequence isonly transmitted in the downlink direction. Each base station transmitsa pilot-sequence on each of the frequencies assigned to that particularbase station as well as on some or all of the frequencies that are notassigned to that particular base station. If the frequency has beenassigned to the base station, the corresponding pilot-sequence istransmitted with slightly more power than the other channels in the basestation. When the base station is transmitting a pilot-sequence on afrequency that has not been assigned to the base station, the basestation just transmits the pilot-sequence at a power level similar toother pilot-sequences.

As illustrated in FIG. 3, each traffic channel 31 and a pilot-sequence33 are combined by multiplication with a unique PN-sequence 34. Thetraffic channels and the pilot-sequence are combined by addition inadder 35 and the resulting signal 36 is modulated in a modulator 37 andamplified in an amplifier 38 and is subsequently transmitted from thebase station 30 through antenna 39.

Since the pilot-sequence signals are received at each mobile stationalong with other signals containing speech information intended for therespective mobile station and noise, each mobile station will have touse a subtraction demodulation process to sort through all of thereceived signals to detect and decode the desired signals. As a result,each mobile station can receive its own signal on its own frequency andsimultaneously measure the signal strengths of all of the neighboringbase stations on the same frequency. Furthermore, the measured signalstrengths can be stored in a memory in the mobile station and updatedperiodically. As a result, each mobile station can send the stored basestation signal strength information to their respective base stations atregular intervals or when requested to do so during Mobile AssistedHandoff.

An exemplary CDMA communication system with subtractive demodulation isdisclosed in U.S. patent application Ser. No. 07/628,359, filed Dec. 17,1990, and U.S. patent application Ser. No. 07/739,446, filed Aug. 2,1991, which is a continuation-in-part application thereof, both of whichare incorporated herein by reference. In the present system, the abilityto tolerate an increased number of interfering signals to therebyachieve an increase in system capacity, is provided through the use of asubtractive demodulation process. Generally speaking, a receiver in thistype of system does not operate to decode only a single desired signalin the presence of a large number of interfering signals. Rather, anumber of received signals, both interfering and desired, aresuccessively decoded in the order of their received signal strengthwherein the strongest received signal is decoded first. After beingdecoded, each interfering signal is recorded and subtracted from thereceived signal, to thereby reduce the interference that is present whenthe desired signal is decoded.

With this approach, a larger number of signals, each having a uniquePN-sequence or scrambling code to provide a means of discriminating themfrom one another, are permitted to overlap. In the followingembodiments, either a PN-sequence or scrambling codes can be used. Insome communication systems, each base station has a set of PN-sequencesor scrambling codes which are assigned to mobile stations, while inother systems each mobile station has its own PN-sequence or scramblingcode. The capacity of such a system is not limited by theoreticalbounds, but rather by the amount of the signal processing resources thatare available to demodulate a multiplicity of signals.

Subtractive demodulation will now be described in conjunction the signalgraphs shown in FIGS. 4-6 which set forth example waveforms in thecoding and decoding processes involved in traditional CDMA systems.Using these same waveform examples from FIGS. 4-6, the improvedperformance of the present invention over conventional CDMA isillustrated in FIG. 7.

Two different data streams, shown in FIG. 4 as signal graphs (a) and(d), represent digitized information to be communicated over twoseparate communication channels. Signal 1 is modulated using a high bitrate, digital code unique to signal I as shown in signal graph (b). Forpurposes of the present invention, the term "bit" refers to one digit ofthe information signal. The term "bit period" refers to the time periodbetween the start and the finish of the bit signal. The term "chip"refers to one digit of the high rate coding signal. Accordingly, thechip period refers to the time period between the start and the finishof the chip signal. Naturally, the bit period is much greater than thechip period. The result of this modulation, which is essentially theproduct of the two signal waveforms, is shown in the signal graph (c).In Boolean notation, the modulation of two binary waveforms isessentially an exclusive-OR operation. A similar series of operations iscarried out for signal 2 as shown in signal graphs (d)-(f). In practice,of course, many more than two coded information signals are spreadacross the frequency spectrum available for cellular telephonecommunications.

Each coded signal is used to modulate a RF carrier using any type ofmodulation technique, such as Quadrature Phase Shift Keying (QPSK). Eachmodulated carrier is transmitted over an air interface. At a radioreceiver, such as a cellular base station, all of the signals thatoverlap in the allocated frequency bandwidth are received together. Theindividually coded signals are added, as represented in the signalgraphs (a)-(c) of FIG. 5, to form a composite signal waveform.

After demodulation of the received signal to the appropriate basebandfrequency, the decoding of the composite signal takes place. Signal Imay be decoded or de-spread by multiplying the received composite signalin the signal graph (c) with the unique code used originally to modulatesignal 1, as shown in the signal graph (d). The resulting signal isanalyzed to decide the polarity (high or low, +1 or -1, "1" or "0") ofeach information bit period of the signal.

These decisions may be made by taking an average or majority vote of thechip polarities during one bit period. Such "hard decision" makingprocesses are acceptable as long as there is no signal ambiguity. Forexample, during the first bit period in the signal graph (f), theaverage chip value is +0.67 which readily indicates a bit polarity +1.Similarly, in the third bit period, the average is +0.80 which indicatesa bit polarity of +1. However, whenever the average is zero asillustrated in the second bit period, the majority vote or averagingtest fails to provide an acceptable polarity value.

In ambiguous situations, a "soft decision" making process must be usedto determine the bit polarity. For example, an analog voltageproportional to the received signal after despreading may be integratedover the number of chip periods corresponding to a single informationbit. The sign or polarity of the net integration result indicates thatthe bit value is a +1 or -1.

The decoding of signal 2, similar to that of signal 1, is illustrated inthe signal graphs (a)-(d) of FIG. 6. After decoding, there are noambiguous bit polarity situations.

Theoretically, this decoding scheme can be used to decode every signalthat makes up the composite signal. Ideally, the contribution ofunwanted, interfering signals is minimized if the digital spreadingcodes are orthogonal to the unwanted signals. Two codes are orthogonalif exactly one half of their bits are different. Unfortunately, only acertain number of orthogonal codes exist for a finite word length.Another problem is that orthogonality can be maintained only when therelative time alignment between signals is strictly maintained. Inmoving constantly, such as in cellular systems, time alignment isdifficult to achieve.

When code orthogonality cannot be guaranteed, noise-based signals mayinterfere with the actual bit sequences produced by different codegenerators, e.g., the mobile telephone. In comparison with theoriginally coded signal energies, however, the energy of the noisesignals is usually small. The term "processing gain" is often used tocompare relative signal energies. Processing gain is defined as theratio of the spreading or coding chip rate to the underlying informationbit rate. Thus, the processing gain is essentially the spreading ratio.For example, a one kilobit per second information rate modulated by aone megabit per second coding signal has a processing gain of 1000:1.

Large processing gains reduce the chance of decoding noise signalsmodulated using uncorrelated codes. For example, processing gain is usedin military contexts to measure the suppression of hostile jammingsignals. In other environments, such as cellular systems, processinggain refers to suppressing other, friendly signals that are present onthe communication channel with an uncorrelated code. In the context ofthe present invention, noise includes both hostile and friendly signals.In fact, noise is defined as any other signals other than the signal ofinterest, i.e., the signal to be decoded. Expanding the exampledescribed above, if a signal-to-interference ratio of 10:1 is required,and the processing gain is 1000:1, conventional CDMA systems have thecapacity to allow up to 101 signals to share the same channel. Duringdecoding, 100 of the 101 signals are suppressed to 1/1000th of theiroriginal interfering power. The total interference energy is thus100/1000 or 1/10 as compared to the desired information signal energy ofone (1). With the information signal energy ten times greater than theinterference energy, the information signal may be correlatedaccurately.

Together with the required signal-to-interference ratio, the processinggain determines the number of allowed overlapping signals in the samechannel. That this is still the conventional view of the capacity limitsof CDMA systems may be gleaned by reading, for example, "On the Capacityof a Cellular CDMA System," by Gilhousen, Jacobs, Viterbi, weaver andWheatly, IEEE Transactions on vehicular Technology, May 1991.

In contrast to the conventional view, an important aspect of the presentinvention is the recognition that the suppression of friendly CDMAsignals is not limited by the processing gain of the spread spectrumdemodulator as is the case with the suppression of military type jammingsignals. A large percentage of the other signals included in a received,composite signal are not unknown jamming signals or environmental noisethat can not be correlated. Instead, most of the noise, as definedabove, is known and is used to facilitate decoding the signal ofinterest. The fact that most of these noise signals are known, as aretheir corresponding codes, is used in the present invention to improvesystem capacity and the accuracy of the signal decoding process.

Rather than simply decode each information signal from the compositesignal, the present invention also removes each information signal fromthe composite signal after it has been decoded. Those signals thatremain are decoded only from the residual of the composite signal.Consequently, the existence of signal transmissions in thecommunications channel from the already decoded signals do not interferewith the decoding of other signals. For example, in FIG. 7, if signal 2has already been decoded as shown in the signal graph (a), the codedform of signal 2 can be constructed as shown in the signal graphs (b)and (c) and subtracted from the composite signal in the signal graph (d)to leave coded signal 1 in the signal graph (e). Signal 1 is recapturedeasily by multiplying the coded signal 1 with code 1 to reconstructsignal 1. It is significant that had the conventional CDMA decodingmethod been unable to determine whether the polarity of the informationbit in the second bit period of signal 1 was a +1 or a -1 in the signalgraph (f) of FIG. 5, the decoding method of the present invention wouldeffectively resolve that ambiguity simply by removing signal 2 from thecomposite signal.

To further facilitate an understanding of the invention, a specificexample is described where a Walsh-Hadamard (128, 7) block codingtechnique is employed to provide channel coding and spreading in CDMAmodulation. However, the principles of the present invention are notlimited to communication systems which employ this encoding technique.In addition, the block codes can be either orthogonal block codes orbi-orthogonal block codes.

An overall view of a CDMA based cellular radio telephone system, of thetype in which the present invention can be implemented, is illustratedin block diagram form in FIG. 8. In this Figure, a transmitter 51 and areceiver 52 are depicted in block form. The transmitter might be presentat a base station of the radio telephone communications system, and thereceiver could be located in a mobile unit, for example. Alternatively,the transmitter could be that of a mobile unit with the receiver locatedin a base station.

Referring to FIG. 8, speech which is generated by one of theparticipants in a telephone conversation is provided as an input signalto a speech encoder 54. The speech encoder can be a conventional encoderwhich converts the speech signal into a digital signal according to anyof the well known types of speech digitizing algorithms. Examples ofsuch algorithms which are employed in conventional speech encodersinclude Continuously Variable Slope Delta Modulation (CVSD), AdaptiveDelta Pulse Code Modulation (ADPCM), Residual Excited Linear PredictiveCoding (RELP) and Vector Code Book Excited Linear Predictive Coding(VSELP). The particular type of encoder that is selected in a givenapplication will depend upon various design factors, such as the desiredcompromise between bit rate reduction and encoder cost and complexity.

After the speech signal has been digitized in the encoder 54 itsbandwidth is expanded to produce a CDMA signal in CDMA encoder 55. Inthe preferred implementation, the CDMA bandwidth expansion is obtainedby means of (128,7) orthogonal block encoding. In addition to blockencoding the digitized speech signal with the block codes, thescrambling device 56 also scrambles the encoded signal with a uniquecipher code that is assigned to the communication. The encryption can,for example, consist of the bitwise modulo-2 addition of a uniquescrambling code to the block code before transmission. The selection anduse of scrambling codes are described in co-pending U.S. patentapplication Ser. No. 07/866,865, filed on Apr. 10, 1992, for "MultipleAccess Coding for Mobile Radio Communications", which is expresslyincorporated here by reference. Since all communications preferablyemploy the same block codes to expand their bandwidth, the scrambling ofthe encoded signals with the unique cipher codes enables the variouscommunications to be distinguished from one another, as described ingreater detail in the previously mentioned copending patent applicationswhich are incorporated by reference.

Once the digitized speech signal has been encoded with the block codeand scrambled with the cipher code, it is passed to a parallel to serialconverter 58. In this circuit, the scrambled speech signal is convertedinto a serial signal that is provided to a modulator 60. A carriersignal at a suitable carrier frequency F_(c) is modulated with thescrambled speech signal, amplified in an amplifier 62, and transmittedto the receiver 52 of the other participant in the conversation.

At the receiver 52, which can be located in a mobile unit for example,the transmitted signal is received, demodulated to remove the carrierfrequency in a demodulator 64, and reconverted into parallel form in aserial to parallel converter 66. The received signal is then unscrambledin a descrambling circuit 68 that is provided with the same cipher codethat was used to scramble the signal. Once the signal has beenunscrambled, it is provided to a Fast Walsh transform circuit 70 thatdetermines which of the possible 128 bit orthogonal code words wastransmitted. In operation, the Fast Walsh transform circuit 70simultaneously computes the correlation of the received code word witheach possible code word, and determines the code word having the highestcorrelation. This determination is carried out in a signaldiscrimination circuit 72. A Fast Walsh Transform and a Maximum SearchCircuit are described in co-pending U.S. patent application Ser. No.07/735,805, filed on Jul. 25, 1991, for "Fast Walsh TransformProcessor", and U.S. patent application Ser. No. 07/761,380, filed onSep. 18, 1991, for "Maximum Search Circuit", both of which are expresslyincorporated here by reference. The signal discriminated code word isthen provided to a speech decoder circuit 74, which converts it into theoriginal speech signal. The signal strength of the received signal canbe stored in a memory 76.

In addition to the desired signal pertaining to the conversation ofinterest, the receiver 52 also receives signals pertaining to otherconversations. For example, the receiver in a mobile unit receivessignal broadcast from the base station to all of the other mobile unitswithin the cell. In essence, these other received signals constitutenoise relative to the desired signal pertaining to the conversation ofinterest. In a preferred implementation of the present invention, theseother signals are also individually descrambled and decoded, in theorder of their received signal strength. Once each of these "noise"signals is determined, it can then be rescrambled and subtracted fromthe original received signal, to thereby reduce interfering noise andfacilitate decoding of the desired signal.

In another embodiment of the present invention, discontinuoustransmission (DTX) and discontinuous reception (DRX) are used to allow amobile station to measure the signal strength of signals on frequenciesother than the frequency the mobile station is presently operating on.FIG. 9 illustrates a typical speech pattern in a communication system.As illustrated, a typical speech pattern of a telephone conversationconsists of periods of speech activity 40 intermingled with periods ofsilence 42 or no speech activity.

An example of a speech frame in a communication system is illustrated inFIG. 10. At the beginning of each speech frame 45, code signals 47called "DTX FLAGS" can be inserted before the speech information section49 to indicated whether the rest of the speech frame contains speechinformation. In this example, code signals A indicate to a receiver thatspeech information will follow, while code signals B indicate to thereceiver that the transmitter is going to discontinue transmission forthe remainder of the speech frame. As a result, a receiver candiscontinue processing that signal for the remainder of the speech frameand regularly measure the signal strength of neighboring base stations.

The present invention provides a communication system which utilizes"discontinuous transmission" (DTX) in a manner that enables receiversynchronization to be maintained while increasing system capacity, andis therefore particularly well suited for use in CDMA communicationssystems. To this end, according to one aspect of the present invention,a speech frame structure is deliberately imposed on the speech encodingmethod, even for those encoding methods which are inherentlystructureless. The speech signal is examined for the presence or absenceof active speech. If no active speech is detected during the duration ofan entire frame, the transmission of the frame of digitized speech codewords is inhibited.

As another feature of the invention, a receiver only attempts todemodulate the received signal for a limited number of sequential codewords. If the signal is not observed to reach a minimum threshold ofcorrelation with a valid code sequence, no further attempts atdemodulating that signal are carried out for the remainder of thepredetermined time corresponding to the speech frame.

Further in accordance with this aspect of the invention, the speechframe structure of a multiplicity of overlapping CDMA signalstransmitted from the same base station are given a fixed relative timealignment. This alignment of the signals allows mobile receivers thatare decoding at least one signal to accurately anticipate when othersignals, that have been temporarily silenced through discontinuoustransmission, are likely to resume transmission. Thus, receiversynchronization and frame alignment information can be obtained fromsignals other than the specific information signal destined for thereceiver.

Preferably, the time alignment relationship employs a fixed pattern ofoffset between the different signals. This arrangement causes the timesat which the different signals can resume transmission to be evenlydistributed over the period of a speech frame. Thus, the times at whichthe receiver attempts to demodulate the various signals is alsodistributed to avoid undesired peaks in receiver activity. According toa further feature of the invention, the speech frame timing fortransmissions from a mobile transmitter is derived from the speech frametiming of signals it receives from the base station. Thus, the relativetiming that the base station chooses for transmissions from the basestations to the mobile receiver is reflected in the relative frametiming between mobile transmissions to the base station, therebyproviding the base station receiver with the benefits of staggered framealignment. An exemplary CDMA communication system is disclosed in U.S.patent application Ser. No. 07/866,555, filed Apr. 10, 1992, entitled"Discontinuous CDMA Reception" and is incorporated herein by reference.

When a receiver receives a signal from a transmitter which indicatesthat the transmitter is going to discontinue transmission for theremainder of the speech frame, the receiver can change frequencies tomeasure the signal strength of neighboring base stations. The measuredsignal strengths can be stored in a memory in the mobile station andupdated periodically. As a result, each mobile station can send thestored base station signal strength information to their respective basestations when requested to do so during Mobile Assisted Handoff.

While the invention has been described in its preferred embodiments, itis to be understood that the words that have been used are words ofdescription rather than of limitation, and that changes within thepurview of the present claims may be made without departing from thetrue scope of the invention and its broader aspects.

What is claimed is:
 1. A method of communication in a code divisionmultiple access system containing a plurality of base stations and aplurality of mobile stations comprising the steps of:dividing saidplurality of base stations into separate groups of base stations basedupon their transmission power levels; assigning each group of basestations a set of frequencies to transmit on, wherein base stations ofdifferent groups do not share frequencies for traffic communication;transmitting a pilot sequence from each base station on all frequenciesassigned to each base station and at least one of the frequencies notassigned to each individual base stations; receiving pilot sequencesfrom neighboring base stations at each mobile station; measuring thesignal strength of each received pilot-sequence; and storing themeasured signal strengths for the neighboring base stations.
 2. A methodof communication in a code division multiple access system according toclaim 1, further comprising the step of:regularly transmitting thestored signal strengths to a base station.
 3. A method of communicationin a code division multiple access system according to claim 1, furthercomprising the step of:transmitting the stored signal strengths to arequesting base station.
 4. A method of communication in a code divisionmultiple access system according to claim 1, further comprising thesteps of:framing speech activity of a speech digitizer in a base stationtransmitter into fixed frames corresponding to a fixed whole number ofbits; generating an activity indication for each fixed frame containingspeech activity; discontinuing the transmission of a base stationtransmitter when there is no activity indication and resuming thetransmission of the transmitter only at fixed frames identified by theactivity indication; deciding whether the transmitter has or has notdiscontinued transmission only at predetermined fixed times whichcorrespond to frames of speech bits at the input of a speech decoder ina mobile station receiver; regularly changing frequencies whendiscontinuous transmission is detected; measuring signal strength ofsignals received from neighboring base stations on other frequenciesafter discontinued transmission has been detected; and storing themeasured signal strength of the neighboring base stations.
 5. A methodof communication in a code division multiple access system according toclaim 1, where different frequency sets are separated by a frequencyguard band.
 6. A method of communication in a code division multipleaccess system according to claim 4, further comprising the stepof:regularly transmitting the stored signal strengths to a base station.7. A method of communication in a code division multiple access systemaccording to claim 4, further comprising the step of:transmitting thestored signal strengths to a requesting base station.
 8. An improvedradio communication system including at least one transmitter, having aspeech digitizer for speech activity, for transmitting code divisionmultiple access radio communication signals having a spread spectrumcode; and at least one receiver having a speech decoder, for receivingthe code division multiple access radio communication signals,comprising:means for framing the speech activity of the speech digitizerinto fixed frames corresponding to a fixed whole number of bits; meansfor generating an activity indication for each fixed frame having speechactivity; means for discontinuing the transmission of the transmitterwhen there is no activity indication and for resuming the transmitteronly at fixed frames identified by the activity indication; means fordeciding whether the transmitter has or has not discontinuedtransmission only at predetermined fixed times which corresponds toframes of speech bits at the input of the speech decoder; means forregularly changing frequencies when discontinuous transmission isdetected; means for measuring the signal strength of signals fromneighboring base stations after discontinued transmission has beendetected; and means for storing the measured signal strengths of thebase stations.
 9. A method of communication in a code division multipleaccess system containing a plurality of transmitters and a plurality ofreceivers comprising the steps of:framing speech activity of a speechdigitizer in a transmitter into fixed frames corresponding to a fixedwhole number of bits; generating an activity indication for each fixedframe containing speech activity; discontinuing the transmission of atransmitter when there is no activity indication and resuming thetransmission of a transmitter only at fixed frames identified by theactivity indication; deciding whether the transmitter has or has notdiscontinued transmission only at predetermined fixed frames whichcorrespond to frames of speech bits at the input of a speech decoder ina receiver; regularly changing frequencies when discontinuoustransmission is detected; measuring signal strength of signals receivedfrom neighboring base stations after discontinued transmission has beendetected; and storing the measured signal strength of the neighboringbase stations.
 10. A method of communication in a code division multipleaccess system according to claim 9, further comprising the stepof:regularly transmitting the stored signal strengths to a base station.11. A method of communication in a code division multiple access systemaccording to claim 9, further comprising the step of:transmitting thestored signal strengths to a requesting base station.
 12. A method ofcommunication according to claim 9 wherein the transmitter is located ina cellular base station and the receiver is located in a mobile station.